Within packaging technology, use is often made of packages of a single-use disposable nature for packing and transporting foods. One very large group of these single-use disposable packages is produced from a packaging material comprising a layer of paperboard and outer, liquid-tight coatings of plastic. In order to impart to the packaging material superior tightness properties vis-à-vis gases, in particular oxygen gas, the packaging material is supplemented with at least one additional layer of a material possessing such tightness properties, normally an aluminium foil (Alifoil).
From the prior art packaging material, packages are produced using modern packing and filling machines which, from a web or from prefabricated blanks of the packaging material, both form, fill and seal finished packages.
From, for example, a web, packages are produced in that the web is first reformed into a tube by both longitudinal edges of the web being folded towards and united to one another in a mechanically strong and liquid-tight sealing joint (“overlap joint”). The tube is filled with an optional food and is divided into cushion-shaped packaging units by repeated transverse sealing of the tube transversely of the longitudinal axis of the tube and beneath the filling level of the tube. The packaging units are separated from one another by incisions in the transverse sealing zones and are given the desired geometric outer configuration, normally parallelepipedic, by an additional fold forming and thermosealing operation.
Correspondingly, packages are produced from flat-folded tubular blanks of the packaging material. The flat-folded blank is raised to an open packaging carton whose one end (e.g. the bottom end) is sealed by fold forming and thermosealing of the continuous foldable bottom panels of the packaging carton. The packaging carton thus provided with a bottom is filled with optional food, whereafter the open end of the packaging carton (in this example the top end) is sealed by fold forming and thermosealing of the corresponding foldable top panels of the packaging carton.
On the market today, there occur such packages of a single-use disposable nature in an almost insurmountably large multiplicity of packaging variations for an almost equally insurmountably large multiplicity of foods, from liquid to solid and semi-solid foods.
One large group of commercial single-use disposable packages comprises so-called aseptic packages in which a previously sterilized food is packed in a similarly previously sterilized package under aseptic conditions. The aseptic package distinguishes itself in that the food may be stored in the package for up to months and even longer at ambient temperature, without the food deteriorating or being ruined.
Another group of prior art single-use disposable packages comprises packages in which the food must, throughout its entire storage time in the package, be kept refrigerated (at most approx. +8° C.) in order not to deteriorate or be completely ruined before its “best before date”.
A further group of such single-use disposable packages comprises so-called retortable packages which are intended to be filled with food and, after sealing, be subjected to a heat treatment for the purposes of extending shelf-life at elevated temperature in an atmosphere of high relative humidity. Like the aseptic packages, the retortable packages have the capability of being able to store the packed food for several months and even longer at ambient temperature without the food deteriorating or being ruined before its “best before date”.
A packaging material of the type described by way of introduction is produced in a per se known manner in that a web of paper is unwound from a magazine reel. The web is led to a printing station where the one side of the web is provided with optional decorative artwork of printing ink and, at the same time, is provided with a corresponding pattern of crease lines in register with the decorative artwork.
Since paper generally displays an extremely coarse and uneven surface characteristic which impairs the printability of the paper surface, and thereby makes it difficult to ensure a good and even quality of decorative artwork, the paper surface is right from the outset provided with a smooth coating which improves printability. Examples of such printability-improving coatings are well-known to a person skilled in the art and in general consist of a combination of pigment and adhesive.
The web provided with decorative artwork and crease lines is led further to a lamination station where both sides of the web are provided with outer, liquid-tight coatings of plastic by extrusion. One typical example of a prior art such extruded coating is polyethylene.
If the packaging material is intended for a package for particularly oxygen gas-sensitive foods, such as juice, wine or cooking oil, the web is moreover provided with a layer serving as oxygen gas barrier, normally an aluminium foil, which is laminated to the web between the paper layer of the web and one of the two outer, liquid-tight coatings.
The laminated web is led further to one or more mechanical processing stations where the web is cut into suitable dimensions and detected faults on the web are removed, before the web is finally wound up into finished, customer-adapted transport reels.
In the prior art method, packaging material is produced for both aseptic packages and for packages in which a packed food must be kept refrigerated throughout its entire storage life up to the moment of consumption (“best before date”). On the other hand, attempts by such means to produce a packaging material for so-called retortable packages have largely failed. One serious drawback which has been observed in connection with a package of the packaging material being subjected to a heat treatment at elevated temperature in an atmosphere of high relative humidity (such as in a steam retort) is that the decorative artwork of the package becomes seriously damaged under these conditions and has thereby rendered the appearance of the package unsightly and unattractive. Not only have cracks and crack formation related to humidity and heat in the decorative artwork carrying coating of the packaging material been noted, but also the mechanical firmness and stability of the package has seriously deteriorated and rendered the package sloppy and difficult to handle. These problems are further aggravated in those cases when the retortable package, during the heat treatment for extending shelf-life is at the same time subjected to high pressure in a retort.
There is thus a need in the art to modify, in a simple manner and by simple means, the method described by way of introduction to such an extent that it may also be employed for producing packaging material for retortable packages without accompanying problems of the type described above. One particular need is to be able to utilise the excess capacity which exists in already established production plants for producing packaging material also for retortable packages, without overly excessive and costly modifications being necessary to these plants.